This invention relates to a process for reducing and eliminating nitrogen oxides in a waste gas which contains the nitrogen oxides and which is emitted from various industrial plants and processes such as a combustion furnace, a nitric acid-producing plant, a fertilizer manufacturing plant and a metal pickling process, thereby to render the waste gas innocuous.
An exhaust gas generated by the combustion of various fossil fuels contains very small quantities of nitrogen oxides NO.sub.x, for example, NO (nitrogen monoxide), NO.sub.2 (nitrogen dioxide), N.sub.2 O.sub.4, N.sub.2 O.sub.5 and N.sub.2 O, which provide for air pollution. Most of nitrogen oxides in the exhaust gases of petroleum fractions (such as naphtha, kerosene, light oil and heavy oil), coal etc. are nitrogen monoxide NO, and a variety of denitrating methods are being developed at present. As one of the methods of eliminating NO, there is a process in which NO is decomposed into nitrogen and water in the presence of a catalyst by employing ammonia NH.sub.3 as a reductant.
The reductive reaction of NO with NH.sub.3 has been considered to proceed according to the following reaction formula: EQU 3NO+2NH.sub.3 .fwdarw.(5/2)N.sub.2 +3H.sub.2 O (1)
It has recently been verified by the inventors, however, that in the presence of oxygen the reduction proceeds according to the reaction of the following formula: EQU NO+NH.sub.3 +1/4O.sub.2 .fwdarw.N.sub.2 +(3/2)H.sub.2 O (2)
(Refer to Official Gazette of Japanese Unexamined Published Patent Application No. 51-56284). As catalysts for promoting the reaction (2), many have been proposed. There can be mentioned, for example, catalysts of mixed oxides containing vanadium (Japanese Unexamined Published Patent Application No. 49-122473); catalysts with molybdenum, tungsten and vanadium added to tin oxides (Japanese Unexamined Published Patent Application Nos. 50-71564 and 51-45689); and catalysts, some of the inventors having participated in the inventions thereof, which contain titanium oxides as their principal constituents and in which are added oxides of iron (Japanese Examined Published Patent Application No. 52-6954), copper, nickel and cobalt (Japanese Examined Published Patent Application No. 52-6953), molybdenum (Japanese Unexamined Published Patent Application No. 50-89291), tungsten (Japanese Unexamined Published Patent Application No. 50-89264), vanadium (Japanese Unexamined Published Patent Application No. 50-128680), uranium (Japanese Unexamined Published Patent Application No. 51-21568), chromium (Japanese Unexamined Published Patent Application No. 51-68473), cerium (Japanese Unpublished Patent Application No. 50-117753), etc. Excepting the catalysts which contain titanium oxides as their principal constituents, alumina or silica alumina is generally used as a catalyst support. In case of the treatment of an exhaust gas containing sulfur oxides, the catalysts employing the alumina type support have the catalytic activity quickly degraded because the alumina turns into aluminum sulfate. The catalysts containing titanium oxides as their principal constituents to which the oxides of Fe, Cu, Ni, Co, Mo, W, V, U, Cr and Ce are added in a range of, for example, at most 50 weight % are not vitiated by the sulfur oxides. Besides, their activities for the reaction of NO and NH.sub.3 are very excellent.
With the catalysts as stated above and under ideal conditions, approximately 90% or more of NO.sub.x can be eliminated from within the exhaust gas. However, when the temperature of the exhaust gas is 250.degree. C. or below, or when the space velocity is increased, the rate of eliminating NO.sub.x lowers, and problems are involved in the application to a large-scale actual plant. If the range of effective reaction temperature can be expanded to a lower temperature region, it will become unnecessary to preheat the exhaust gas to the optimum temperature, which is economically advantageous. If the reaction rate can be raised, the reaction will proceed with a smaller quantity of catalyst, and the required power for transferring the exhaust gas will decrease, which also enhances economy.